1. Field of the Invention
The invention relates to a screw drive with rolling elements and, more particularly, to a ball screw drive including a screw spindle with a spindle axis and a double nut running on the screw spindle with two separately formed single nuts which are substantially rigidly joined in the direction of the axis of the spindle. In order to set a desired play or bias relative to the screw spindle, the two single nuts can be turned continuously about the axis of the spindle relative to each other and fixed in any relative rotary position corresponding to the desired play or bias.
2. The Prior Art
Screw drives of the foregoing type are used, for example, for rapid and accurate positioning of tool and work holders and for moving them at defined speeds. Rotary motion of the screw spindle is converted to linear motion of the guided double nut. In order to satisfy the highest possible demands for accuracy in reversing the direction of motion of the double nut, the double nut has to be arranged on the spindle as far as possible without play. For this purpose, when the two nuts are being mounted on the spindle they are turned towards each other far enough to make their facing end surfaces bear against each other. Further turning of the nuts causes the rolling elements to be applied to opposing side surfaces of the raceways of the nuts, which ultimately results in the desired biasing of the two nuts relative to the screw spindle and guidance of the double nut on the spindle substantially without play.
A screw drive of the above type, i.e., a screw drive in which the play or bias of the two single nuts relative to the screw spindle can be infinitely adjusted, is known, for example, from DE-OS 21 35 812. A screw bolt is contained in an intermediate element of the known screw drive which is non-rotatably joined to one of the single nuts, the screw bolt being in screw engagement with a part mounted non-rotatably on the other single nut. The worm drive thus formed allows infinitely variable turning of the two single nuts relative to each other about the axis of the spindle. One disadvantage of this construction is that a relatively large amount of space has to be provided for the worm drive, particularly in a radial direction. Another disadvantage is that, in spite of the self-locking inherent in worm drives, the desired bias set may undesirably re-adjust itself, for example, due to vibrations, causing the screw bolt of the worm drive to turn.
A similarly constructed screw drive is known from DE-OS 23 49 958.
Versions of the generic screw drive are known from DE-OS 23 49 958 and from DE 30 38 774 C2 and the corresponding EP 0 049 903 B1 in which an intermediate element is in frictional engagement with both single nuts in order to prevent relative rotation between them. The frictional forces must therefore be overcome during the assembly, i.e., the relative turning of the two single nuts to set the desired bias or play. This makes exact setting of the play or bias to the desired value considerably more difficult.
Screw drives are further known from DE 24 53 635 C3, DE-OS 25 33 996 and DE 29 40 762 C2 in which the two single nuts can only be turned relative to each other and fixed in predetermined angular steps rather than infinitely variably.
In the screw drive known from German utility model 77 08 184, infinitely variable turning of the two single nuts is only possible within a very narrow angular range. In order to set a desired bias or play relative to the screw spindle, it is therefore also necessary to provide spacers between the two nuts, and their thickness must be determined by the desired bias or play. Hence, even with experienced operators it is usually necessary to change the spacers several times during assembly, until the spacer with the right thickness for the desired bias or play has been found. Assembly of the screw drive known from DE-GM 77 08 184 is therefore laborious and time consuming.
In DE 39 00 693 C2, a cavity formed by annular grooves and indentations between the two touching end faces of the single nuts is filled with casting compound to form the intermediate element. The position of relative rotation of the two single nuts corresponding to the desired bias or play must therefore be maintained until the casting compound has completely hardened.
A screw drive where the two single nuts are joined by an intermediate element which yields resiliently in the direction of the axis of the spindle is known from DE 42 08 126 A1. An axially resilient arrangement of this type has the drawback that, when a force exceeding the spring force is exerted on one of the single nuts, the two single nuts may move towards each other, thereby losing the desired bias.
In DE 32 07 566 A1, after the desired bias of the two single nuts has been set, a hole to receive a radial locking bolt is formed in their common contact region, to ensure that the bias obtained is maintained.
An object of the invention, therefore is to provide a screw of the abovementioned type which, with a simple construction and simple assembly, readily enables the play or bias of the single nuts relative to the screw spindle to be adjusted accurately and infinitely variably to a desired value.
According to the invention, the foregoing object is attained by the provision of a screw drive of the above type, in which either a first of the two single nuts or an intermediate element joined thereto and arranged between the two single nuts is welded to the second single nut or to at least one part joined thereto, or a first of the two single nuts or an intermediate element joined thereto and arranged between the two single nuts has at least one peripheral section pressed into an associated aperture in the second single nut. In accordance with the invention, the non-rotational joining of the first single nut or the intermediate element to the second single nut is the last operation in assembling the screw drive according to the invention. The possibly necessary non-rotational joining of the intermediate element to the first single nut or the formation of the intermediate element on the first single nut may take place in a preceding operation or simultaneously with the non-rotational joining of the intermediate element to the second single nut.
The limit between direct joining of the two single nuts and joining an intermediate element integral with one of them to the other single nut is a fluid transition in practice. It is not always possible to decide which of the two extreme cases applies. Basically any part of the single nut used for joining the other single nut may be considered as an intermediate element, even when that intermediate element is not a separately formed part of the single nut in question.
In order to join the two single nuts or the intermediate element non-rotatably to the second single nut, the latter is first brought into contact with the first single nut or the intermediate element by turning on the screw spindle. The desired play or bias of the two single nuts relative to the screw spindle is then set by further turning of the nuts. The only frictional forces arising between the intermediate element and the nut emanate from the actual bias. When the play or bias has reached the desired value, then as a first alternative the first single nut or the intermediate element is welded to the second single nut or to a part joined thereto, or as a second alternative a peripheral section of the first single nut or of the intermediate element is pressed into an associated aperture in the second single nut. In both cases, a secure, non-rotational joint is obtained, allowing immediate further processing of the screw drive. And in both cases subsequent accidental turning of the two single nuts relative to each other, with resultant re-adjustment of the desired play or bias, is reliably avoided.
The use of weld connections in the screw drive according to the invention and in the method of the invention will be described in detail at a later stage.
To enable the first single nut or the intermediate element to be separated from the second single nut, the first single nut or the intermediate element may be welded to at least one fit component which is detachably fixed to the second single nut, for example being screwed to it by means of a screw bolt. When the screw bolt has been released, the fit component may, for example, be levered off the second single nut with a tool, thus breaking the weld between the fit component and the first single nut or intermediate element. When the broken location has been smoothed, the single nuts and intermediate element can be re-used without any problems and re-welded to another fit component when the play or bias has been set to the desired value. Alternatively, the double nut may be transferred to an auxiliary spindle with a smooth surface, i.e., an auxiliary spindle without a helically extending raceway. When the screw bolt has been released, the second single nut can be pulled off the fit component welded to the first single nut or the intermediate element in an axial direction on the smooth auxiliary spindle without the weld having to be broken.
To facilitate the above-mentioned levering of the fit component off the second single nut, the inside diameter of the hole passing through the fit component may be larger than the outside diameter of the screw bolt, and the hole passing through the fit component may be provided with an internal screw thread. In that case, a screw bolt with an outside diameter larger than that of the screw bolt used for fixing the fit component to the second single nut may be driven into the threaded hole. As this "release bolt" is screwed further and further into the hole it levers the fit component off the second single nut. This process takes place in an extremely controlled manner owing to the strong transmission action emanating from the thread. This is advantageous particularly when the fit component lies substantially flush against the first single nut or the intermediate element along a predetermined distance, and is preferably welded to the first single nut or the intermediate element along that entire distance.
Alternatively to welding to a fit component, it is also possible for the first single nut or the intermediate element to be welded to the second single nut along at least part of--preferably along substantially the whole of--the contact line, visible from the outside, between those components. In that case, however, it is not possible to dismantle the first single nut or the intermediate element from the second single nut without breaking them. Yet this embodiment is particularly suitable for applications with a low risk of damage, owing to the small number of parts and the consequent opportunity for cost-effective production and assembly.
In another alternative embodiment, the peripheral section may be pressed into the aperture in the second single nut in an axial or/and radial direction. It is then advantageous for the peripheral section which is pressed in to bear on the wall surfaces bounding the aperture only in a peripheral direction. In this way, one can reliably avoid forces acting in either an axial or a radial direction that might influence the bias or play of the two single nuts relative to the screw spindle.
In order to reduce the impressing forces required or/and to enable the peripheral section pressed in to be applied to the side walls of the aperture over the largest possible area, provision may further be made for the peripheral section which is pressed in to be indented.
To increase the stability of the whole arrangement and thus reduce the risk of deformation of the nuts, particularly in their end regions facing towards each other, the aperture may be bounded radially inwardly by a wall section of the second single nut.
In certain applications it may be advantageous for the first single nut or the intermediate element to be additionally adhered to the second single nut, preferably using a double-sided adhesive tape. Adhesive tapes with curing synthetic resins suitable for this application are obtainable, for example, under the name of SCOTCH.TM. VHB.TM..
Also, the second single nut may clearly be joined to the first single nut or the intermediate element by means of a plurality of fit components welded to the first single nut or intermediate element, or a plurality of sections of the first single nut or intermediate element pressed into apertures in the second single nut may be spread over the whole periphery.
As already indicated above, there are various possible ways of joining the intermediate element to the first single nut. Thus, the intermediate element may both be integral with the first single nut and an element separate from that nut. In the first case, a particularly stable non-rotational connection is obtained between the intermediate element and the first single nut, although it makes it difficult to dismantle the double nut of the screw drive on the screw spindle without breaking it.
In cases where the intermediate element is a separate element, there are again various ways of fixing it non-rotatably to the first single nut:
For example, the intermediate element may be joined to the first single nut with a press fit. Additionally or alternatively, the intermediate element may be adhered to the first single nut, preferably using a double-sided adhesive tape, as already mentioned for joining the intermediate element to the second single nut.
To obtain a positive connection between the intermediate element and the first single nut, the intermediate element may be held non-rotatably relative to the first single nut by at least one additional fit component, which is joined to or integral with one of the two components, i.e., the intermediate element or the first single nut, and which in either case engages in an aperture provided in the other component, the first single nut or the intermediate element.
In a first alternative form of the positive connection, the additional fit component may be formed by a peripheral section of the intermediate element which is pressed into an aperture in the first single nut. In a second alternative form, it is also possible for the additional fit component to be a separate component and to be fixed detachably to the first single nut, for example, by being screwed onto it by means of a screw bolt. The advantages of these two alternative joining methods may be seen from the above discussion of the various ways of joining the intermediate element to the second single nut. In particular, if an additional fit component which can be screwed onto the first single nut is used, it is then possible for the inside diameter of the hole passing through the additional fit component to be larger than the outside diameter of the screw bolt, and for the hole to be provided with an internal screw thread.
Another possible way of joining the first single nut to the intermediate element is to have a plurality of additional fit components distributed around the whole periphery.
Another method of joining the intermediate element to the first single nut is to screw it onto or into the nut, for example, by means of a fine thread. The decisive factor in making the joint between the intermediate element and the first single nut non-rotational in operation is that the pitch and/or rotary direction of the screw connection between them should be different from those of the screw spindle.
Finally, it is also possible for the intermediate element to be welded to the first single nut along at least part of--and preferably along substantially the whole of--the contact line, visible from the outside, between the two components.
To facilitate reproducible and accurate assembly of the screw drive according to the invention it is proposed, in a further embodiment, that the intermediate element on at least one of the single nuts, and preferably both single nuts, be arranged centered in respect of the axis of the spindle, or that the two single nuts be arranged centered relative to each other in respect of the axis of the spindle.
To increase the stability of the screw drive but also to ensure high-precision manufacture, it is further proposed that the intermediate element should be formed by a closed ring member.
To obtain a screw drive according to the invention which can be produced at a favorable cost, it is also possible for the intermediate element to be formed by a slotted ring member, which may, for example, be formed by a sheet metal strip bent into a ring shape. The two end regions of the metal strip adjoining the slot may be fixed to at least one of the two single nuts by welding, in order to obtain a ring member which is substantially rigid in operation. The slotted ring member may be fixed cost-effectively to at least one of the two single nuts, e.g., by spot welding. It is preferable for the slot in the intermediate element to be closed by welding.
To allow a certain amount of tolerance adjustment, the intermediate element may have an elastic region which allows the two single nuts to tilt relative to each other about axes substantially perpendicular to the axis of the spindle, while joining the two single nuts substantially rigidly in an axial direction, i.e., particularly an elastic region which to a limited extent allows only swivelling movements of the two single nuts relative to each other about axes extending perpendicular to the axis of the spindle.
Further according to the invention, at least one of two components to be joined, i.e., the first single nut or intermediate element, or the second single nut or intermediate element, or one of the two single nuts, may be made of soft, i.e., non-hardened, steel at least in a region determined for joining to the respective other component. These soft regions may also be treated in a simple manner by the end user. For example, tapped holes may be made in these soft regions with ordinary commercial tools, allowing highly flexible installation of screw drives according to the invention in higher-level structures. Moreover, the soft regions of one component may be pressed into associated apertures in the other, to join the two components.
Particularly in view of the above-mentioned possibility of joining the components of the screw drive according to the invention by welding, the soft regions may be made of a weldable steel, for example, a steel with a low carbon content. It has hitherto been assumed in the state of the art that for two components of a screw drive to be welded together both components had to be made of a steel with a low carbon content. However, appropriate tests carried out by applicants have shown that a weld connection of satisfactory quality can be obtained even if only one of the two components is made of a steel with a low carbon content, while the other is made of a steel with a high carbon content. For example, the intermediate element may be made of a steel with a low carbon content while the two single nuts are made of a hard steel with a high carbon content.
This realization is advantageous particularly because in many cases it makes it possible to avoid the use of case-hardened single nuts, i.e., single nuts in which the rolling element raceway is hardened by carburisation--and thus has a high carbon content--and is surrounded by an outer surface region made of a softer steel with a low carbon content. With the above construction, the production of case-hardened nuts requires a relatively large outlay and is thus expensive. For the reasons explained above, it is an independent feature of the invention that two components of a screw drive may be welded together in which only one is made of a steel with a low carbon content.
The use of case-hardened single nuts is nevertheless not excluded. The provision of at least one case-hardened single nut may be appropriate, e.g., when the intermediate element is integral with one of the two single nuts and the nut is to be joined to the other single nut by welding or pressing in. In a case-hardened single nut for this purpose, the region to be welded or pressed in is not carburised. Apart from case-hardening, the spindle raceways of the single nuts may also be induction hardened. This method, in which the steel to be hardened is merely induction heated instead of having carbon supplied to it, is particularly suitable for use in single nuts made entirely of carbon-rich steel. Owing to the high carbon content of the outer surface region surrounding the spindle raceway, induction hardened single nuts cannot be welded directly together. However, the outer surface region remains soft in the induction hardening process, so the single nuts can be joined by the pressing-in method described above.
Laser welding processes, for example, are suitable to form the weld connection. One advantage of such processes is that no material has to be supplied externally during welding, e.g., in the form of welding wire. It is sufficient for the two parts being welded together to lie flush against each other. Another advantage of laser welding processes is that the laser beam can be divided with simple optical means, so that two parallel welds can be made simultaneously. This is beneficial particularly when the intermediate element is non-rotatably joined to each single nut by a weld. When a component made of steel with a high carbon content is welded to a component made of steel with a low carbon content, zones with a varying carbon content form in the weld.
A further advantage of the laser welding process is that the welding depth can be well controlled, so that both the actual weld and all the surrounding area affected by heat can be kept very narrow. In this way, only a comparatively small amount of heat is passed into the nuts during welding, so there is no fear of deterioration of the rolling element raceways or damage to the rolling element diverting members, which are made of plastic and inserted in the nuts at this final stage of assembly. The quality of the weld may optionally be improved by pre-heating or post-heating the region close to it. Post-heating in particular can prevent the critical cooling speed of the weld from being exceeded. In view of the material of the rolling element diverting members, the upper limit for the temperature to be used in their vicinity during pre-heating and/or welding and/or post-heating is approximately 120.degree. C. Higher temperatures can certainly prevail in spots within regions of the single nuts remote from the diverting members. The screw drive can be put back into use immediately after welding. No idle times or expensive post-treatments are necessary.
However it is basically also possible to use other welding methods, e.g., TIG (tungsten/inert gas) welding or soldering.
Regardless of the welding method used, it is preferable for the welding or seam, of which there is at least one, to be in the form of a V-welding or V-seam in order to obtain a smooth external surface for the double nut. It is further advantageous for the two components being welded together to be chamfered in the region envisaged for welding.
To protect the weldings or seams used to join the two single nuts --with the aid of an intermediate element if so desired --from corrosion or similar noxious external influences, it is proposed, further according to the invention, that these weldings or seams should be covered by means of a covering ring. The ring may be made of plastic, metal or other suitable material and may further be fixed on the screw drive, e.g., by adhesion. If a slotted covering ring is employed, it may easily be clipped onto the screw drive and held to the screw drive solely by the elasticity inherent in the ring.
In all the embodiments discussed above, it is firstly possible for the two single nuts to bear directly on each other in the direction of the axis of the spindle. Alternatively, it is also possible for the two single nuts to bear on each other in the direction of the axis of the spindle by means of the intermediate element.
If at least one fit component is employed both when joining the intermediate element to one single nut and when joining it to the other single nut, it is proposed that all the fit components should be identical. This has advantages in respect of the cost of producing and storing the fit components.
The invention further concerns a method of assembling a screw drive according to the invention, particularly a ball screw drive, wherein, in order to set a desired play or bias, the two single nuts are placed against each other by turning them on the screw spindle and are joined non-rotatably in a relative rotary position corresponding to the desired play or bias and wherein, in order to fix the relative rotary position of the two single nuts, the first of the two single nuts or an intermediate element joined thereto and arranged between the two single nuts, as a first alternative, is welded to the second single nut or to at least one part joined thereto or, as a second alternative, has at least one peripheral section pressed into an associated aperture in the second single nut. The advantages of this method and its possible forms are described above in connection with the discussion of the screw drive according to the invention.
Finally, the invention relates to a method of assembling a screw drive, particularly a ball screw drive, wherein, in order to set a desired play or bias by turning on the screw spindle, the two single nuts are placed against each other and joined non-rotatably in a relative rotary position corresponding to the desired play or bias and wherein, when the desired play or bias has been obtained, a common aperture is cut out of the two single nuts and a fit component is inserted and fixed in the aperture. The invention further concerns a screw drive assembled by this method.